Shutter Angle

Expressing shutter in degrees (°) instead of fractions (1/50s) for video. Standard language on film sets.

Rotating disc analogy

Film cameras used a rotating disc shutter with a cutout on part of it. Light only reached the film while the cutout was passing in front of it.

       ┌───┐
      ╱     ╲
     │  ●●● │     ← Half (180°) of the disc is the cutout.
     │       │       As the camera rotates once,
      ╲     ╱        light hits the film for half that time.
       └───┘

• 180° open = exposed for half of one frame's duration
• 360° open = exposed for the full frame duration (theoretical maximum)
• 90° open = exposed for 1/4 of one frame's duration

Conversion to fractions

Shutter speed = 1 / (fps × 360 / shutter angle)

Or: Shutter angle = (shutter speed × fps × 360) °

Why degrees, not fractions

One reason matters most.

"180°" guarantees the same look at any frame rate.

At 24fps, 180° = 1/50. Bump to 60fps. Keep 1/50 and you'd be at 360° — far too much motion blur. At 60fps, 180° is 1/120.

In fractions you recompute every time fps changes. In degrees you say "stay at 180°" and you're done. That's why cinema cameras (ARRI, RED, Sony Venice) ship with shutter expressed in degrees as a built-in option.

How different angles look

The classic 90° example is the Normandy beach landing in Saving Private Ryan. Shorter exposure removes motion blur, each frame becomes photo-sharp, and the chaos of explosion and gunfire gets that nerve-jangling feel.

Summary

• Shutter angle and shutter speed are the same thing in different notation.
• Sets use degrees because they're fps-independent.
• 180° became standard because, since the 24fps era, that amount of motion blur looked most natural to the human eye.

Related

• frame-rate-and-shutter-angle — Parent note
• shutter-speed — Shutter as time
• frame-rate